In a color image forming process, image data corresponding to an input image is encoded in a color space of tristimulus values i.e. RGB (red, green, blue) or another input color space. Each combination of color values represents a unique color in the image, and each picture element (“pixel”) in the image is represented by a combination of the primary colors of the input color space. To form and print a color output image associated with a color input image, a charged photoconductive imaging member may be sequentially exposed to a series of color separated images corresponding to the primary colors in an input image in order to form a plurality of color separated latent images. Each color separated image is developed with a complementary developing material (toner) containing a primary color or a colorant, such as cyan, magenta and yellow. The complementary developing material used to develop each color separated image is the subtractive complement of the respective color separated image. The developed color separated images are subsequently superimposed, in registration, on one another to produce a multicolor image output. Thus, a multicolor image is generated from patterns of different primary colors or their subtractive complements which are blended by the eye to create a visual perception of a color image.
Each color separated image produced by separating and superimposing color images comprises an arrangement of pixels corresponding to a spot to be developed with toner particles of a particular color. With half-toning, the multicolor image is a mosaic of different color pixels, wherein the color separations are laid down in the form of halftone dots. In halftone image processing, the dot densities of each of the color components making up the multicolor image can be altered to produce a large variation of color hues and shades. For example, lighter tints can be produced by reducing the dot densities such that a greater amount of white from the page surface remains uncovered to reflect light to the eye. Likewise, darker shades can be produced by increasing the dot densities. This method of generating process color images by overlapping halftones of different colors corresponding to the primary colors or their subtractive equivalents is well known in the art.
A printer which has an ideal dye behavior has a one-to-one correspondence of cyan-to-red, magenta-to-green, and yellow-to-blue. This means that when printed, the cyan ink will only absorb red light, the magenta ink will only absorb green light, and the yellow ink will only absorb blue light. However, inks typically have a non-ideal dye behavior and, therefore, have a complex non-linear colorimetric response because interactions between the cyan, magenta, and yellow inks exist which result in unwanted absorptions of reds, greens, and blues. Therefore, the amount of a complementary developing material, such as toner, used in forming an image is not always proportional to the level of the respective input color.
In pigment-based and dye-based applications, a Color Rendition Dictionary (CRD) is provided as a guide for mixing colorants to achieve a desired target color. The CRD comprises a multi-dimensional look-up table reflecting the gamut or possible range of colors that can be made by combining the primary colors. Each of the three axes of the table defines a respective primary color. For example, for an RGB color space, the entry having coordinates (255, 0, 0) corresponds to fully saturated red, similarly (0, 255, 0) corresponds to green, and (0, 0, 255) corresponds to blue. The entry (0, 0, 0) corresponds to pure black and the entry (255, 255, 255) corresponds to pure white, with a range of color combinations in between. Each entry of the CRD contains a formula or “recipe” for creating the particular color defined by the coordinates of the entry. The formula specifies the relative amounts of primary colorants, typically but not restricted to Cyan, Magenta, Yellow, and Black to mix in order to create a desired color. The Color Rendition Dictionary is incorporated in an image processor, such as a Digital Front End (DFE). The DFE receives input image data and converts the image data to a pixel image so that the printing system may produce a “hardcopy” of the image. The CRD instructs the printing system to produce a specified amount of each respective complementary developer material to create the desired colors in the output image.
Even after a Color Rendition Dictionary is formed in the memory of a printing system, the relationship between the input colors and the toner combination used to print the color may drift over time. This is due to the tendency of the pigments of the toner to vary and change. Therefore, after a printer is calibrated, a formula for creating a color that comprises an entry of the CRD may not accurately produce the correct color. In other words, the colors asked to be printed and the actual colors printed are not the same. Periodic recalibration of the color mixing process to ensure accurate reproduction of color is necessary in certain color image forming processes, particularly where a very specific customer selectable color is required for trademarks, logos and the like. As such, the customer is usually highly concerned that the color meets particular color specifications.
In copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of prints is to artificially create a “test patch” of a predetermined desired density. Data for process controls algorithms are typically obtained by reading test patches on the photoreceptive imaging member, usually in the interdocument zone (IDZ). The interdocument zone comprises a portion of the photoreceptive imaging member that is not used to form an output image. The actual density of the developer material in the test patch can then be optically measured to determine the effectiveness of the printing process.
For example, U.S. Pat. No. 5,543,896 discloses a method for measurement of tone reproduction curves using a single structured patch for providing development control by storing a reference tone reproduction curve and providing a single test pattern including a scale of pixel values in an interdocument zone on a photoreceptor surface. The test pattern is sensed in the interdocument zone and a control response to the sensing of the test pattern is provided with reference to the tone reproduction curve in order to adjust the machine operation for print quality correction.
U.S. Pat. No. 5,450,165 to Henderson et al discloses a system for identifying areas in pre-existing image data as test patches for print quality measurement. The system described therein is used to screen for printing data consistent with an area in a visible image having predetermined density condition. These “incidental” test patches comprise a portion of the developed image on the photoconductive imaging member or on the output media.
U.S. Pat. No. 5,471,313 to Thieret et al., uses a control system for an image output terminal with a hierarchical structure which isolates subsystem controls for purposes of efficient algorithm design, analysis and implementation. The architecture is divided into three levels and has a controls supervisor which provides subsystem isolation functions and reliability assurance functions.
While the controls systems of the prior art have been effective in monitoring and controlling single color separations, these control systems are inadequate for monitoring other print quality factors, such as color mixing. The interdocument zone in a printing system is inadequate for a controls system designed to stabilize a color mixing process. To provide effective control, information for the range of colors output by a printing system must be obtained. There is inadequate space in the IDZ to provide the large amount of color data that is required. Moreover, test patches created and measured on the photoconductive imaging member, both in the IDZ and within the image data, do not acknowledge contributions to color made by media, transfer and fusing systems.
Prior controls systems for a color mixing process involve complicated and time-consuming external processes to calibrate and control the color printing process. In addition to wasting customer consumables, such as toner and paper, these external processes significantly reduce the productivity of a printing system, as the printing system is unusable for creating images during the calibration process.